Cryogenic liquid transport pipelines are used to move super cooled liquidized fluids such are liquid natural gas (LNG). A primary purpose of such a pipeline is to transport super cooled liquidized fluids between large land based storage containers located at a port terminal facility and transport ships in a harbor which carry the liquidized fluids to other ports. Because the draft of a transport ship may be in excess of sixty feet, the ship often must remain a substantial distance from the shore and from the liquidized fluid storage containers. Thus, in some instances cryogenic liquid transport pipelines must be of substantial length which may be in excess of several miles. Such pipelines are constructed of a series of individual spools that are welded together. The spools may be eighty feet or more in length and have diameters of between four inches and thirty-six inches. The pipelines may extend above ground, under ground and/or underwater.
In order to maintain the super cooled condition of a cryogenic fluid within a cryogenic liquid transport pipeline, the pipeline spools are constructed with a central axially extending mainline pipe which carries the super cooled liquidized fluid and a concentric secondary containment pipe from two to six inches larger in diameter than the mainline liquid fluid transport pipe that overlies and is welded to the exterior of the mainline liquid transport pipe. The individual spools are welded together in a series to form a cryogenic liquid transport pipeline of desired length. At each welded spool joint a concentric secondary containment pipe is welded to the adjacent secondary containment pipes for the spools to provide a secondary joint containment space at the spool joint. The secondary containment space for each spool and the secondary joint containment space for the spool joints are evacuated to provide a vacuum insulation for the individual pipe spools and for the pipe spool joints to maintain the super cooled condition of the cryogenic fluid within the liquid transport pipeline.
Secondary containment of a vacuum insulated cryogenic liquid transport main pipe requires special safety measures. Typically the pressure of a cryogenic fluid in a transport pipe is between 5 and 400 pounds per square inch. If a small leak develops in a cryogenic main liquid transport pipe causing cryogenic liquid fluid to pass into an evacuated secondary containment space it will vaporize rapidly when exposed to an ambient temperature environment. This will cause the fluid volume and pressure in the secondary containment space to rapidly increase and possibly cause a catastrophic failure of the pipeline.
There are several methods of protecting a secondary containment space from over pressurizing and causing a failure of the pipeline. One method is to add a relief valve to the secondary containment pipe jacket which would vent gas in the secondary containment space to atmosphere when it reached a set pressure. Of course, this results in a loss of fluid and may be undesirable in some pipeline locations and situations. A second method is add a relief valve to the secondary containment pipe jacket and connect the outlet of the valve to a low pressure manifold that has a common relief device for all pipe spools. This method is undesirable for a number of reasons. It necessitates having external pipes that extend parallel to the pipe spools. This is undesirable inasmuch as should a pipe spool roll during handling or be handled roughly, the external pipe and it attendant relief valves may be damaged. Additionally, passing cryogenic fluid into a low pressure vent manifold will cause the relief valves of the adjacent spools to cool to a very low temperature, which may cause them to leak and thus cause multiple spool failures. Thus, it is desirable to provide a system for protecting the secondary containment space of a cryogenic liquid transport pipeline spool from over pressurizing, that contains any leaks from the cryogenic mainline liquid transport pipe within the secondary containment pipe space of the failed spool, prevents external leaks of fluid to the outside environment, reduces the rate of leakage from the mainline liquid transport pipe and does not require connection to adjacent pipeline spools.